Dangerous debris from the collapsed Interstate 35W bridge and rough conditions in the Mississippi River continue to slow underwater divers in their hunt for victims in Minneapolis, but it turns out the elements have also prevented response teams from using the newest high-tech rescue equipment in the first place. In at least one instance, officials at the wreck sight rebuffed an offer of sea-seeking robots from nearby authorities, while the shallow water may be presenting problems of its own.

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Most underwater search efforts rely upon the sonar scanners already being used in Minneapolis, but robotics have played an increasingly important role in scouting for survivors and wreckage. Eight-pound, joystick-controlled swimming 'bots called Remote Operating Vehicles (ROVs) can transmit live-feed video to a rescue boat's computer screen, acting as birddogs before recovery teams send scuba divers into potentially deadly debris conditions. "ROV's are very safe and save a lot of time," says John Sanders, director of the National Underwater Rescue and Recovery Institute. "This is the type of equipment you need."

But just as powerful eddies, dangerous rebar spears and murky waters have delayed and even temporarily halted human recovery efforts at the I-35W site, standard ROV cameras can turn up very little in such turbid conditions. "There are times when an ROV works great and times when it doesn't," says Dave Phillips, undersherriff for Minnesota's St. Louis County, who offered his ROV supply to the I-35W rescue effort but has not received a response.

And while any ROV would likely be crushed by a piece of falling wreckage or a direct impact from jutting rebar, super high-frequency sonar can mount on a different types of ROVs to offer more detailed images. "It's so powerful we could read the brand name printed on the bottom of the sole of a shoe," says Steve Saint-Amour, deep water operations director of Phoenix International, which the Navy contracts for all its underwater rescue and recovery efforts.

Given that 6-in. visibility has reduced them to relying on Braille-like reconnaissance, Minneapolis fire department divers have instead mounted side-scan sonar detectors directly onto boats. The scanners send a fan of sound waves into the Mississippi, then measure the resulting echoes in order to form a kind of topographical map of the area. But at about 7-feet-deep, the river's shallow bed presents another challenge to divers: Sound can bounce off of the water's surface, interfering with sonar readings and further slowing the search process.

With recovery efforts pushing into a third day, local officials considered bringing in barges and cranes to remove larger sections of steel and concrete, opening up space below the debris and clearing an already treachorous path for recovery workers, who are being as high-tech as they can be.

"The divers will be taking extreme caution," Hennepin County sheriff Rich Stanek said today. "We will be slow and methodical during our search operations." —Jancy Langley, with reporting by Julianne Pepitone